1. Field of the Invention
The present invention relates to a gas sensor and a gas detection method for determining a gas concentration, useful in various fields of semiconductor manufacturing, food production/manufacturing/distribution, medical care, etc., and more specifically to improved gas sensor and gas detection method suitable for detecting an extremely small amount of a gas.
2. Description of the Related Art
In a field of manufacturing of semiconductor devices such as LSIs (large scale integrated circuits), several hundred ppb (parts per billion) of an impurity is purposefully added to control a semiconductor characteristic. Therefore, the residual oxygen content of a high-purity gas used in this field has to be at a several ppb level (i.e. 1 to 10 ppb level), 2 digits smaller than the several hundred ppb. However, there are still no gas sensors sensitive to the several ppb level. For this reason, an expensive analysis equipment is used in this field.
In a food-related field, it is expected that if a food putrid odor can be detected at a 10 to 50 ppb level, equal to the human olfaction level, food freshness management can be automatized.
Furthermore, in a medical-related field, it is expected that detection of a metabolite from a sickness person (such as acetone from a diabetic patient, methylmercaptan from a periodontitis patient, a volatile sulfur compound, triethylamine, or ammonia from a liver disease patient, carbon monoxide from an asthma patient, or isoprene from an overcholesterol patient) can contribute to early medical diagnosis. In addition, in a deodorant (antiperspirant) field, application of the high sensitivity detection to self check of body odor is under consideration. These fields require a high detectivity at a 1 to 50 ppb level.
As described above, a gas sensor having a concentration sensitivity at the several ppb level can be used in various fields. Conventional gas sensors include semiconductor type gas sensors and surface acoustic wave type gas sensors. For example, JP-A-2000-121588 discloses a gas sensor having a WO3 thin film doped with In2O3, SnO2, or Sb2O5 on a gold electrode. An electrical characteristic of the electrode is measured when trimethylamine comes into contact with the thin film. The tungsten oxide has a high sensitivity to the living thing putrid odor causing substance (trimethylamine), has a perovskite-like crystal structure partly, and further has a dielectric characteristic. JP-A-2001-305089 discloses a gas sensor having a metal oxide semiconductor of tin oxide. The semiconductor is doped with both a pentavalent transition metal and a trivalent transition metal, so that oxygen in the air is stably adsorbed to a surface of the semiconductor.
However, the semiconductor type gas sensor has a low detectivity at 1 ppm level. The surface acoustic wave type gas sensor is not suitable for repetitive operation requiring a concentrated gas. In any case, the gas sensors described in JP-A-2000-121588 and JP-A-2001-305089 cannot achieve the high detectivity at the ppb level.